But last time I checked, this was a free country, and people should presumably be free to express their opinions

The fact that this is a "free country" means that you're free from government interference with your right to free speech, not that you're guaranteed a forum to say whatever you like. The moderators of the site have the right to not give you the space to say something if they don't want to. That has nothing to do with the concept of free speech, or the fact that it's "just an opinion." If you don't like the way the moderators moderate the site, make your own site. It's a free country, after all.